The Government has rights in this invention pursuant to Contract F33657-83-C-0281 awarded by the Department of the Air Force.
This invention relates to thermal barrier coating systems and, in particular, to a modified intermediate metallic coating structure between the substrate and the thermal barrier coating.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to 1900.degree.-2100.degree. F.
Many approaches have been used to increase the operating temperature limit of the turbine blades and vanes. The composition and processing of the materials themselves have been improved. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cool air is forced through the channels during engine operation.
In another approach, a thermal barrier coating system is applied to the turbine blade or turbine vane component, which acts as a substrate. The thermal barrier coating system includes a ceramic thermal barrier coating that insulates the component from the hot exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component. Ceramic thermal barrier coatings usually do not adhere well directly to the superalloys used in the substrates. Therefore, an additional metallic layer called a bond coat is placed between the substrate and the thermal barrier coating. The bond coat is usually made of a nickel-containing overlay alloy, such as a NiCrAIY or a NiCoCrAIY, of a composition more resistant to environmental damage than the substrate. The bond coat may also be made of a diffusional nickel aluminide or a platinum aluminide, whose surface oxidizes to a protective aluminum oxide scale.
While superalloys coated with such thermal barrier coating systems do provide substantially improved performance over uncoated materials, there remains room for improvement. Improved environmental resistance to destructive oxidation and hot corrosion is desirable. In some instances, the alloying elements of the bond coat can interdiffuse with the substrate alloy to produce brittle intermetallic phases, and avoidance of such phases would be desirable.
There is an ongoing need for an improved procedure for manufacturing an improved article protected by a thermal barrier coating system, wherein the environmental resistance and long-term stability of the thermal barrier coating system is improved. The present invention fulfills this need, and further provides related advantages.